Recently, demands of a capacitor using an electrode material made of active carbon as memory backup power supply, auxiliary power supply or the like have rapidly increased and the capacitor has attracted special interest with the development of the field of electronics. Also it has recently been required to perform further size reduction of the memory backup power supply and to develop a capacitor having a high energy density capable of performing heavy current charge/discharge in a moment at a high capacity used in applications such as solar backup energy storage device and power source adapted for vehicle.
A conventional capacitor can be roughly classified into two types according to a difference in charge storage mechanism. One is a so-called electric double layer capacitor utilizing an electric double layer capacity accumulated at the interface between an active carbon electrode and an electrolytic solution, while the other one is a redox capacitor utilizing an pseudo double layer developed by the redox reaction on the surface of an electrode made of a metal oxide.
It has been studied to use the electric double layer capacitor in applications such as power supply for driving motors of electric vehicle, hybrid vehicle or the like, and power supply of regenerative energy devices upon breaking because it has a high power density and high reliability to repeating of a charge/discharge cycle compared with a conventional secondary battery. As the electrode material, active carbon having a large specific surface area of about 1000 to 2000 m2/g has widely been used in view of chemical stability and high conductivity. It is considered that active carbon containing a large number of pores having a diameter of 2 nm or more is preferably used to perform heavy current charge/discharge because the thickness of the electric double layer is deemed to be about 1 nm.
Recently, a redox capacitor using an electrode made of a noble metal oxide of ruthenium, iridium or the like has been proposed to increase the capacity of the capacitor. The redox capacitor enable charges to accumulate due to adsorption/desorption of hydrogen ions to the metal oxide electrode. As the electrode material, a metal oxide or pi-conjugated conductive polymer, wherein valences vary with incoming and outgoing of d-electrons, is used.
However, the capacitor has not enough capacity and heavy current characteristics to use in applications such as energy storage device or power source adapted for vehicle and high capacity and high performances are required to both the electric double layer capacitor and the redox capacitor.
It is acknowledged that the electric double layer capacitor is markedly superior in power density and cycle characteristics to the secondary battery. However, a higher energy density is required in a power supply of hybrid vehicle, electric vehicle or the like.
The redox capacitor using a noble metal oxide of ruthenium, iridium or the like has advantages such as high energy density, small size and low internal resistance, and less danger of ignition as compared with the electric double layer capacitor. However, practical higher capacity is required when using in heavy current applications. Ruthenium and iridium are very expensive because of low production, and a drastic study is required in view of the manufacturing method, mass-producibility and cost.